1. Field of the Invention
This invention relates to devices containing conductive polymer compositions.
2. Summary of the Prior Art
Conductive polymer compositions, and devices comprising them, are known. Reference may be made for example to U.S. Pat. Nos. 2,978,665 (Vernet et al.), 3,221,145 (Hager), 3,243,753 (Kohler), 3,311,852 (Rees), 3,351,882 (Kohler et al), 3,448,246 (Armbruster), 3,535,494 (Armbruster) 3,571,777 (Tully), 3,691,349 (MacColl et al), 3,793,716 (Smith-Johannsen), 3,823,217 (Kampe), 3,861,029 (Smith-Johannsen), 4,017,715 (Whitney et al), 4,085,286 (Horsma et al), 4,135,587 (Diaz), 4,177,376 (Horsma et al), 4,177,446 (Diaz), 4,188,276 (Lyons et al) 4,237,441 (Van Konynenburg et al) and 4,246,468 (Horsma); U.K. Pat. No. 1,534,715; the article entitled "Investigations of Current Interruption by Metal-filled Epoxy Resin" by Littlewood and Briggs in J. Phys D: Appl. Phys, Vol. II, pages 1457-1462; the article entitled "The PTC Resistor" by R.F. Blaha in Proceedings of the Electronic Components Conference, 1971; the report entitled "Solid State Bistable Power Switch Study" by H. Shulman and John Bartho (August 1968) under Contract NAS-12 -647, published by the National Aeronautics and Space Adminstration; J. Applied Polymer Science 19, 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18, 649-653 (1978) Narkis et al; and U.S, Ser. Nos. 750,149 (Kamath et al), now abandoned, published as German OLS No. 2,755,077; 732,792 (Van Konynenburg et al), now abandoned, published as German OLS No. 2,746,602; 751,095 (Toy et al), now abandoned, published as German OLS No. 2,755,076; 798,154 (Horsma et al), now abandoned, published as German OLS No. 2,821,799; 965,344 (Middleman et al), now U.S. Pat. No. 4,238,812; 965,345 (Middleman et al), now U.S. Pat. No. 4,315,237; and 6,773 (Simon), now U.S. Pat. No. 4,255,698. For details of more recent developments in this field, reference may be made to U.S. Ser. Nos. 41,071, (Walker), now U.S. Pat. No. 4,272,471 67,207 (Doljack et al) now abandoned, 88,304 (Lutz) now U.S. Pat. No. 4,361,799, 97,711 (Middleman et al), 141,984 (Gotcher et al), 141,987 (Middleman et al) now U.S. Pat. No. 4,315,237 141,988 (Fouts et al), 141,989 (Evans), 141,990 (Walty), now U.S. Pat. No. 4,314,231 141,991 (Fouts et al), 142,053 (Middleman et al), now U.S. Pat. No. 4,352,083, 142,054 (Middleman et al), now U.S. Pat. No. 4,317,027, 150,909 (Sopory), 150,910 (Sopory), now U.S. Pat. No. 4,334,351 and 150,911 (Sopory), now U.S. Pat. No. 4,318,881 and the application Ser. No. 364,179 filed on Apr. 2, 1981, by Jacobs et al (MP0762). The disclosure of each of the patents, publications and applications referred to above is incorporated herein by reference.
Many of the electrical devices comprising conductive polymers make use of generally planar electrodes, and for each electrodes the use of foraminous electrodes, especially metal mesh electrodes, has been most generally recommended and found to be of practical value in order to achieve adequate physical adhesion between the conductive polymer and electrode, coupled with low contact resistance. However, the use of foraminous electrodes inevitably leads to some degree of electrical non-uniformity; furthermore if the surface of the electrode closest to the other electrode has any imperfections, this can lead to electrical stress concentration which will cause poor performance. This problem is particularly serious when the conductive polymer exhibits PTC behavior, since it can cause creation of a hot zone adjacent the electrode; it also becomes increasingly serious as the distance between the electrodes gets smaller. Ser. No. 141,990 (Walty) describes planar electrodes which are layers of flame-sprayed metal; such layers can be produced either by flame-spraying the metal directly onto the conductive polymer or by laminating the conductive polymer with a layer of metal previously sprayed onto a carrier, e.g. a film. The possibility of using metal foil electrodes in heating devices comprising conductive polymers is also disclosed in the prior art. For example U.S. Pat. Nos. 3,448,246 (Armbruster) and 3,535,494 (Armbruster) discloses planar heaters which comprises a planar conductive polymer element, e.g. a PTC element, which is sandwiched between two metal foils, each preferably 10 to 25 microns thick. Such foils, according to U.S. Pat. No. 3,535,494 can be applied to the element in any convenient manner. However, there is no description in either patent of any specific procedure in which the foils were united to the element, or indeed of any specific conductive polymer element. U.S. Pat. No. 3,221,145 (Hager) discloses large area (generally at least 1.times.4 feet) electric heaters which comprise a planar conductive polymer element which is sandwiched between two metal foils, each of thickness 0.0001 to 0.01 inch. The conductive polymer has a resistivity of 4.times.10.sup.3 to 4.times.10.sup.7 ohm.cm and the element is 0.2 to 0.001 inch thick. The method of making such heaters which is disclosed in the patent comprises coating each of the foils with a liquid conductive polymer mix, e.g. a polymeric latex which is then dried, followed by lamination of the two coated foils under heat and pressure. U.S. Pat. No. 3,691,349 (Mac Coll) describes a heating element which comprises a polysiloxane-based conductive polymer element to which metal foil electrodes are secured by eyelets. U.S. Pat. No. 3,311,862 (Rees) discloses heating elements which comprise a planar conductive polymer element which is sandwiched between two metal foils. Rees refers to the difficulty of bonding conductive resinous films to metallic foils, and in order to overcome this difficulty he makes use of a conductive polymer which comprises carbon black dispersed in plasticised polyvinyl chloride and bonds the conductive polymer element to the metallic foils by means of a key coat comprising carbon black dispersed in a resinous binder containing 25-75% of a vinyl chloride/vinyl acetate copolymer and 75-25% of a vinyl chloride/vinyl alcohol copolymer.